In SRE (another on-line game), all players pay taxes to a game
coordinator, and each player gets a portion of the total tax money at
the beginning of each session. This has two effects:
1 - Large players pay more taxes than small ones, but receive
the same amount, so smaller players benefit from the taxes.
2 - New players get an amount of money that somewhat corresponds
to the strengths of the other players. If you have a lot of
old, large empires, then the new players start with more, so
that they have more of a chance.
Another thing that SRE does is that it runs like a video game: on
higher "levels" (corresponding to stronger players), the game is
harder on them. For example, building up military is more expensive
(per unit) if the empire is old and large, and it is very cheap when
the empire is small or new. A new player will start with a lot of
money to buy cheap military units, so that he has a chance. It also
gets harder to play as time goes on, so any one player can't stay at
the top forever.
If you make it easier to run a small empire than a large empire, then
it won't be very easy to stay at the top, unless you're _really_ good.
(As opposed to getting to the top because you were first.)
Having your game victory conditions depend on a ratio of power to
size, as Jonathan Gibbons described, is a great idea! It gives your
new players a chance immediately, rather than having to survive for
some amount of time before they can compete against the others, or only
competing with people that join at the same time as them.
Good luck with your game!
-- /\\ Amit J Patel, firstname.lastname@example.org \\/ <- it's a dilithium crystal